Game Freak is perfecting Pikachu, their goal is to make him as cute as possible. Pikachu is more than a Pokemon he is a public idol and well adored character, and honestly I think they really got his shape in Pokemon X and Y.
I assume this question is coming from a humorous point of view because I've seen many meme's about how Pikachu changes over generations,
So as a humorous response.( Pikachu has been diagnosed with the Kanto syndrome. You will find that many of the Kanto league Pokemon crys/sounds have been alterd to sound more like them instead of the weird pixlated sound they all made in Pokemon Blue, Red and Yellow.) <- Not real answer